1. Field of the Invention
The present invention relates to a power detection circuit, and more particularly to a power detection circuit for surely initializing the internal circuit of a power detection circuit and enabling the repetition operation of a starter circuit when is connected or disconnected while power is switched on (hot pluggable operation) in the power supply of a device for performing a connection/disconnection operation while power is switched (hot pluggable operation) which requires a circuit operation response.
2. Description of the Related Art
FIG. 1 shows the system configuration of the conventional power detection circuit.
The system comprises a power-on detection circuit 1 for detecting the power-on level of a power supply, a power-on latch circuit 2 for maintaining the state after detecting the power-on, a power-down detection circuit 3 for detecting the power-down level of the power supply by power drop after the power supply is started up, or the like, a power detection signal generation circuit 4 for receiving these signals and generating a power detection signal, and a starter circuit 5 for initializing the system when power is switched on.
First, when power is switched on, the starter circuit 5 outputs a signal nrset and the power voltage Vdd crosses the second potential and the power-on detection circuit 1 outputs a signal pwren and the power-on latch circuit 2 latches the power-on state. Then, when the power voltage Vdd crosses the second potential again, the power detection signal generation circuit 4 outputs a signal resetgo. Then, when power drops by ΔV (difference between the second and first potential, that is, hysteresis) after a signal starts up, the power-down detection circuit 3 outputs a signal resetctl, thereby resetting the power-on latch circuit 2.
In the above-described conventional circuit configuration, to respond to the connection or disconnection operation while power is switched on (hot pluggable operation), the power-on latch circuit 2 is initialized using a control signal based on the hysteresis operation from the power-down detection circuit 3 when the power-down is detected. However, since the control signal from the power-down detection circuit 3 is invalid when power is initially switched on, the balance between elements constituting the power-on latch circuit 2 or the like is broken to make the power-on latch circuit 2 enter into a desired state when power is switched on. Specifically, by changing the W/L of two elements constituting the power-on latch circuit 2, one of them is set high and the other is set low at an early stage.
Furthermore, even in the conventional circuit configuration, when power is switched on, it sometimes takes a significant amount of time to define the initial state of a node in the detection circuit by a power waveform. In order to cope with inconveniences, such as the fluctuation of a power detection level and the like, to define the initial state of the node, the starter circuit 5 is included. However, since starter circuit 5 is only activated when power is initially switched on, starter circuit 5 does not function when power is switched on secondly and after it is connected or disconnected while power is switched on (hot pluggable operation). Therefore, in this case, the power-on latch circuit 2 is not compulsorily initialized. As described above, this initialization performs a hysteresis operation by breaking the balance between elements constituting the power-on latch circuit 2.
In this case, since a control signal from the power-down detection circuit is invalid when the power is initially switched on, countermeasures to break the balance between devices constituting the latch circuit or so on are taken to put the latch circuit in a desired state when the power is switched on. However, the above-described countermeasures is not sufficient, depending on the start-up speed of the power supply, the latch circuit 2 is not sometimes initialized.
In order to break the balance between elements, it is necessary to change the W/L between both the elements. In this case, there is waste in the layout of devices and extra space is needed.
Furthermore, since there is a current path even when the power-on detection circuit 1 does not operate, power consumption is wasted.
The above-described conventional configuration is disclosed by patent reference 1.
In order to realize a more stable power-down detection level, the power-down detection circuit disclosed by patent reference 1 in the same form as the power-on detection circuit must be built in, there is a current path even when it does not operate. Therefore, power consumption is wasted. The power-down detection circuit disclosed by patent reference 1 dependently operates using the output of the power-on detection circuit as a trigger, and a hysteresis width sometimes changes.
Patent reference 1: Japanese Patent Application Publication No. 2006-5459 “Reset Circuit”
Patent reference 2: Japanese Patent Application Publication No. 2001-237684 “Power-on Reset Circuit”
The above-described prior art cannot define initialization since the initialization is performed using the hysteresis operation of the latch circuit. Therefore, the connection/disconnection (hot pluggable operation) cannot be stably performed. Furthermore, in order to realize a more stable detection level, the power-down detection circuit is not of a simple type performing the dependent operation of securing only a hysteresis width, as disclosed by patent reference 1. A detection circuit with an independent detection level is needed and the same circuit configuration as the power-on detection circuit must be introduced into a power-down detection circuit. Therefore, even when neither the power-on detection circuit nor the power-down detection circuit operates, there is a current path.